1. Field of the Invention
The present invention relates to circuits for controlling television cathode ray tubes (CRT's), and more particularly to a circuit operable for indicating that the CRT has reached a sufficient warm-up level when a television set is powered.
2. Discussion of the Related Art
FIG. 1 schematically shows a control stage of one of the three cathodes of a color CRT, for example, a red cathode (R). The circuit includes a PNP transistor QR whose emitter is connected to the red cathode (not shown) and whose collector is grounded through a shunt resistor (R.sub.s). The base of transistor QR is controlled by an amplifier 10 which receives a current I.sub.R acting as a red channel signal, and a correction current I.sub.OR for the correction of a black level. The correction current I.sub.OR is provided by a voltage-current converter 12 whose input voltage is the voltage at the terminals of a capacitor C1. The voltage across capacitor C1 is fixed by a differential amplifier 14 which receives at a first input terminal, a reference voltage V.sub.ref and at a second input terminal, the voltage across resistor R.sub.s, i.e., an indication of the cathode current I.sub.K.
Converter 12, capacitor C1, and amplifier 14 form a so-called black level regulation loop. The regulation loop periodically re-adjusts the correction signal I.sub.OR so that the black level of the input signal I.sub.R corresponds with the black level of the corresponding cathode.
The black level regulation loop is enabled, as will be seen in more detail with reference to FIG. 3A, during the duration of a line at each initial portion of the frame and disabled during the remaining duration of the frame. Such a short regulation phase allows for storing in capacitor C1 a correction voltage, changed into the correction current I.sub.OR, serving during the remaining portion of the frame. The possibility of enabling and disabling the regulation loop is symbolized by a switch k1 disposed between capacitor C1 and amplifier 14. The regulation loop is enabled by a signal EN.sub.R that is filtered by an AND gate 16 so that the regulation loop can be enabled only after a warm-up phase of the CRT. The final portion of the warm-up phase is defined by a circuit 18 operable for detecting a warm CRT by measuring the current of cathode I.sub.K. The output terminal 19 of circuit 18 is connected to an input terminal of the AND gate 16.
Circuits similar to those that have been described are associated with a transistor QG operable for controlling the green cathode, and with a transistor QB for controlling the blue cathode. The regulation loops associated with the green and blue cathodes are enabled by respective signals EN.sub.G and EN.sub.B that are provided to input terminals of AND gates 20 and 21. The same resistor R.sub.s serves to measure the sum of the three cathode currents I.sub.KR, I.sub.KG, and I.sub.KB. Similarly, only the detection circuit 18 is operable for determining the end of the warm-up phase and for allowing transmission of the enabling signals EN.sub.R, EN.sub.G, and EN.sub.B.
FIG. 2 is a schematic of a typical circuit 18 for warm-up detection. The voltage across resistor R.sub.s feeds a first input terminal of a comparator 24, a second input terminal receiving the reference voltage V.sub.ref. The output V.sub.c of comparator 24 is applied to the set input S of an RS-type flip-flop 26. The reset input R of flip flop 26 is connected to a reset circuit (RST) 28 at powering. The output V.sub.c of comparator 24 is transmitted to flip flop 26 only during each of the three black level regulation phases, i.e., during the activation of each signal EN.sub.R, EN.sub.G and EN.sub.B, which is represented by a switch k2 disposed between the output of comparator 24 and flip-flop 26 and controlled by an OR gate 29 receiving at its input signals EN.sub.R, EN.sub.G, and EN.sub.B. The role of a low-pass filter, including resistor R2 and capacitor C2, disposed between the shunt resistor R.sub.s and the input of comparator 24, will be described below.
FIG. 3A shows waveforms of various signals during a warm-up phase of the CRT and during a normal phase. A signal H.sub.rt is shown in the form of pulses corresponding to line retraces, and a signal V.sub.rt have long-duration pulses corresponding to frame retraces. Corresponding with signals H.sub.rt and V.sub.rt are the signals enabling the regulation loop EN.sub.R, EN.sub.G, and EN.sub.B ; input signals I.sub.R, I.sub.G, and I.sub.B of the three control circuits of the cathode; and voltage V.sub.R appearing across the shunt resistor R.sub.s.
The pulses of signal H.sub.rt successively occur within a period of 64 microseconds. The pulses are separated by periods of 52 microseconds corresponding to line periods. The line periods are numbered 1, 2, 3 . . . in the vicinity of signal H.sub.rt. A first series of periods 1, 2, 3 . . . on the left-hand portion of the drawing corresponds to a warm-up phase of the CRT; a second series 1, 2, 3 . . . on the right-hand portion of the drawing corresponds to a normal phase. Signals H.sub.rt, V.sub.rt, EN.sub.R, EN.sub.G, and EN.sub.B remain identical during the warm-up and normal phases.
Time t.sub.0 corresponds to an initial portion of a line retrace and to the beginning of a frame. The frame retrace signal V.sub.rt goes from a high state to a low state and remains at the low state for the duration of the frame. Signals EN.sub.R, EN.sub.G and EN.sub.B go through an active state respectively during line periods 1, 2, and 3, and then remain inactive until the next frame.
During a warm-up phase, the enable signals EN.sub.R, EN.sub.G, and EN.sub.B generate measurement windows in which it is determined whether the current of cathode I.sub.K reaches a sufficient level. During a normal phase, these signals generate windows during which are carried out regulations of the black level for each red, green and blue channel.
The current signals I.sub.R, I.sub.G, and I.sub.B provided to the control circuits of the cathode are low level pulses during line retraces, and are at a constant level during the frame retraces. From line 4, the current signals exhibit variations corresponding to the image to be displayed on the screen. These variations are imperceptible on the screen as long as the CRT has not reached its nominal operating condition.
During a warm-up phase, as represented on the left-hand portion of FIG. 3A, the current signals have a relatively high level during the generation of the windows corresponding to the enable signals, for rapidly enabling the CRT. The cathode current progressively and gradually increases during the line periods 1, 2, and 3. At a time preceding the initial portion of the frame shown in the right-hand portion of FIG. 3A, the current of the cathode reaches the black level corresponding to a voltage having a value V.sub.ref across resistor R.sub.s. At this time, the output signal V.sub.c of comparator 24 (FIG. 2) switches to high state and is transmitted, since a measurement enable window is generated, to the set input S of flip-flop 26. Then, flip-flop 26 authorizes the validation of the regulation loops of the black level during the subsequent enable windows.
On the right-hand portion of FIG. 3A (normal phase), each of the current signals has, during a corresponding enable window, a so-called "black level" or reference level. As shown by curve V.sub.R, the cathode current is regulated to a value corresponding to voltage V.sub.ref during the generation of enable windows.
The first lines of a frame, especially lines 1, 2, and 3, are not visible on the screen. This is the reason why the black level regulation and the warm-up phase of the CRT are carried out during these line periods. It is essential that the black level regulation be inhibited during the warm-up phase, because the difference between the actual cathode current and the desired value would cause, during the first frames at powering, high correction currents I.sub.OR, I.sub.OG, I.sub.OB. Such high correction currents would generate unpleasant visual effects.
FIG. 3B shows the actual waveform of voltage V.sub.R across resistor R.sub.s, as well as the corresponding waveform of the output signal V.sub.c of comparator 24. At each initial and final portion of a line period, voltage V.sub.R exhibits a high positive peak. Such voltage peaks, that are present in the measurement windows, can exceed voltage V.sub.ref. As represented, the output signal V.sub.c of comparator 24 switches to a high state in the measurement windows during each voltage peak, whereas the mean voltage V.sub.R has not reached value V.sub.ref yet. Thus, flip-flop 26 may be enabled and the black level regulation loops may start operating prematurely.
The low-pass filter R2-C2 of FIG. 2 is designed to avoid this drawback. However, to sufficiently attenuate the positive voltage peaks, capacitor C2 must have a high value (approximately equal to a few microfarads). The present trend is to integrate most of the elements of the television circuits. Since capacitor C2 cannot be integrated, a specific pin 30 (FIG. 2) of the integrated circuit must be provided to connect capacitor C2. Additionally, capacitor C2 and its mounting involve non-negligible costs.